The present invention relates to a fuel injector.
A fuel injector for a fuel injection system of an internal combustion engine having a piezoelectric actuator is described in German Published Patent Application No. 195 34 445. In addition, the valve has a valve closing body that can be operated by a one-piece valve needle and works together with a valve seat face to form a sealing seat. Fuel is supplied to the valve through a fuel inlet connection, with a fuel line leading to the sealing seat. The actuator is tubular in design, and the valve needle extends through the tubular actuator. The valve needle has an axial longitudinal opening, so that the valve needle forms a section of the fuel line which is thus surrounded by the actuator at least in some sections.
A fuel injector is described in German Published Patent Application No. 19500706. In the fuel injector described in this publication, a valve closing body together with a valve seat face forms a sealing seat. The fuel injector is designed either as an outward opening fuel injector or an inward opening fuel injector. The valve closing body is connected in one piece to a valve needle which can be operated by a piezoelectric actuator. Piezoelectric actuators have a relatively small operating stroke. It is therefore proposed in this publication that a hydraulic step-up device be arranged between the piezoelectric actuator and the valve needle to increase the actuator""s operating stroke.
One disadvantage of these known fuel injectors is their relatively complicated design which has not been optimized for compactness. Another disadvantage is that the special hydraulic medium used for the hydraulic step-up device evaporates over a period of time due to leakage losses, which can have a negative effect on the lifetime and operation of this fuel injector.
A fuel injector having a piezoelectric actuator in another design is described in German Patent No. 43 06 073. However, this design is also not very compact, and it requires a relatively large space for installation. With the fuel injector derived from this publication, a hydraulic step-up device is again provided to transform the relatively small stroke of the piezoelectric actuator to a larger stroke of the valve needle. A special pressurized accumulator, e.g., in the form of an external pressure reservoir, is used to compress the hydraulic medium of the step-up device and to compensate for leakage losses. This requires an additional connection for this pressure reservoir or other measures to implement the accumulator.
Fuel injectors having piezoelectric actuators as known in the past were used mainly to inject fuel into a self-igniting internal combustion engine, in particular for injecting diesel fuel. The operating pressures occurring in this case are relatively high and have so far prevented a more compact, space-saving design.
The fuel injector according to the present invention has the advantage over the related art that it has an extremely compact design due to the fact that the fuel line passes through the tubular actuator. With the known fuel injectors, fuel passes by the actuator at the side, resulting in a wider design, or is supplied downstream from the actuator, which is usually unfavorable for the connection of the fuel line; however, the design according to the present invention yields a centrally guided fuel line with the possibility of providing the fuel inlet connection on the end of the fuel injector opposite the valve closing body. The housing may be designed with a relatively thinner wall, because the fuel line need not pass by the side of the actuator as in the related art.
The compact design also results in a short intake path for fuel, thus preventing cavitation problems.
In an advantageous manner, the valve needle extends through the tubular actuator, a longitudinal axial opening through the valve needle forming a section of the fuel line. The valve needle assumes the function of operating the valve closing body as well as the function of the fuel line.
It is especially advantageous to design the valve needle in two parts connected by a weld or a coupling piece. In this way, a valve needle section on the spray end can be inserted into the fuel injector on the spray end, and a valve needle section on the inlet end can be inserted into the fuel injector on the inlet end, with the two valve needle sections being joined subsequently. A flange providing support for a restoring spring may be premounted on the valve needle section on the inlet end.
The actuator is preferably surrounded radially by a prestress element. In comparison with the axial arrangement of the prestress element which is customary in the related art, this measure results in a more compact design.
It is especially advantageous to use fuel passing through the fuel line as the hydraulic medium for the step-up device. Then a special hydraulic medium such as oil is not needed for the step-up device. Any leakage losses are compensated by an automatic refilling operation. In addition, there is no risk of contamination of the fuel with a different type of hydraulic medium such as hydraulic oil.